A hot 64.1 °C simple thermal spring (hypotonic, weakly alkaline) drawn as a blend led by the Unsenji wellhead. Total dissolved solids are light at 764 mg/kg, but metasilicic acid is generous at 159 mg/kg — enough to leave the thin silica film that makes skin feel noticeably smoother. Weakly alkaline at pH 7.5, the water reads as soft on the skin and is colorless, tasteless, and odorless at the source. Among Japan's hotter natural sources, so cooled before entering the bath.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺173mgP43 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.2mgP35 national9%
- Potassiumminor; matters for drinking limitsK⁺17.5mgP60 national5%
- LithiumLi⁺1.3mgP84 national2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻—mg55%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻142mgP49 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻71.2mgP50 national17%
- Fluoridetrace; drinking limits applyF⁻—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃159mgP75 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.